A filter press is a filter machine that is industrially most widely used at present because of its various advantages. For example, the structure is simple, rugged and is rarely damaged; the machine is easy to handle, and a high filtering pressure can be obtained. A general structure of a diaphragm containing type filter press that is one of the filter presses, is shown in FIG. 1. By way of explaining the structure of this diaphragm-containing type filter press, taking the directions X--X in FIG. 1 as back and forth directions for convenience, fixed plates 3 and 4 are supported respectively on two pedestals 1 and 2 disposed at the front and rear positions and are separated by a predetermined interval. These fixed plates 3 and 4 are connected by a plurality of connecting plates 5. On the connecting plates 5 are mounted a plurality of filter frames 6 so as to be movable in the forward and backward, and on one side of these plurality of stacked filter frames 6, a movable plate 7 is mounted on the connecting plates 5 and also is movable in the backward and forward directions. Fixedly secured to the movable plate 7 is a tip end of a ram 9 of a hydraulic cylinder 8 assembled in the fixed plate 4, so that the movable plate 7 can be driven back and forth by the cylinder 8. The movable plate 7 and the filter frames 6 are connected as by linkage mechanisms (not shown), so that in response to the movement of the movable frame 7 the filter frames 6 may move along the connecting plates 5 with an appropriate interval maintained therebetween. When the filter frames 6 are urged against the fixed plate 3, filter chambers surrounded by filter cloths provided on the filter frame 6 are formed between the respective filter frames 6.
In such a diaphragm-containing type filter press, after the filter frames 6 have been compressed and fastened, at first slurry, that is, a liquid to be processed within the filter chamber, is fed and filtered under pressure for a predetermined period of time, thereafter, the feeding of the slurry is stopped. Subsequently, pressurized water is fed into an interstice between an inflatable diaphragm, such as a rubber sheet or the like, extended over the surface of the filter frame 6 and that filter frame surface to compress and filter the slurry. Finally a dehydrated cake formed within the filter chamber is taken out by opening the filter frames. Upon feeding the slurry, however, a pressure difference is liable to arise between the respective filter chambers because of a difference in concentration of the slurry, contamination of the filter cloths and other causes. This is especially the case when the slurry is liable to be mixed with foreign matters, for example the supply water mud, sewage water mud, or the like. In these instances the feed port for the filter chamber may be blocked by foreign matters, and consequently, the pressure of the slurry feed pump acts upon the partition walls of the filter frames forming the filter chamber as a pressure difference. If a pressure difference occurs between the filter chambers, the partition wall of the filter frame will bend to the side of the filter chamber having a lower pressure, resulting in deformation of the filter frame and damage to the filter cloth. In order to prevent such deformation and damage, heretofore, the wall thickness of the partition wall of the filter frame was made very thick, but as a result, the weight of the machine became heavy, the machine length became long, and a large foundation for the machine was necessary.